The present invention relates, in general, to a solid-state image sensing device and, in particular, to a CCD image sensor which is swing-driven to periodically shift in position relative to incident image light.
In a conventional solid-state image sensor such as an interline transfer type CCD (referred to as an IT-CCD hereinafter), it is easy to understand that the number of picture elements or pixels must be increased to obtain a high resolution video image with the interlaced scanning technique. However, such a conventional IT-CCD with only the standard number of pixels has the largest chip size of common LSIs. If the number of pixels is simply increased in such a CCD to satisfy the need for high integration, its chip size is further increased. On the other hand, in order to increase the number of pixels without changing the present CCD chip size, the integration of pixels must be greatly enhanced (e.g., more than four times). With this approach it is technically difficult to prepare such a high density CCD. Even if development of improved fabrication techniques allows the manufacture of the CCD in the near future, the circuit configuration for driving the CCD becomes complicated and power consumption becomes high, thus entailing new problems.
In order to solve the problem of the number of pixels vs packing density, the CCD may be periodically displaced relative to incident image light, thereby picking up a high resolution image. In two field periods (A and B) constituting one frame period in the NTSC system, when the CCD is swung at an amplitude corresponding to 1/2 the pixel pitch (PH) such that pixels are positioned in different sampling positions along a horizontal direction, the spatial sampling points along the horizontal direction can be increased. Therefore, the horizontal resolution of the one-frame image can be substantially doubled even if a conventional low resolution IT-CCD chip is used.
With the above swing image pickup technique, however, the resolution of the reproduced image can be improved but the generation of moire cannot be reliably suppressed. This moire generation results in noise, which reduces the quality of the image.
The frequency of the moire effect becomes nearly zero near the threshold of the Nyquist rate causing a rough noise pattern in the reproduced image. Furthermore, when field images picked up by this swing image pickup technique are composed and a single frame image is constructed, the phases of the moire in both field images are mutually reversed so the effect of the noise is increased. Also, because the CCD chip is physically vibrated in relation to the incident image light, as was described above, a moire effect is transferred onto the screen, which was not produced with the prior art image sensor which picks up images in a stationary state. This transfer of the moire effect onto the screen results in deterioration of the image quality and, accordingly, the generation of the moire effect is a major problem with fixed imaging apparatus, which perform swing image pickup.